Support loading extensions into the bookmark toolbar.

chrome/test/data/extensions/good/extension1/1/manifest.json

 {
   "format_version": 1,
   "id": "00123456789ABCDEF0123456789ABCDEF0123456",
   "version": "1.0.0.0",
   "name": "My extension 1",
   "description": "The first extension that I made.",
+  "toolstrip": "toolstrip.html",
   "content_scripts": [
     {
       "matches": ["http://*.google.com/*", "https://*.google.com/*"],
       "js": ["script1.js"]
     },
     {
       "matches": ["http://*.yahoo.com/*"],
       "js": ["script2.js"]
     },
     {
       "matches": ["http://*.news.com/*"],
       "js": ["js_files/script3.js"]
     }
   ]
 }